The present instant invention relates to a drive unit and, in particular, to a drive unit for loading equipment into an airplane.
Equipping airplanes with automatic or semi-automatic loading equipment to load and unload containers or pallets (Unit Load Device "ULD") is known. For this purpose conveyor rollers in U-rails are normally installed on the floor of the loading deck for conveying of the material to be loaded in a straight line into the interior cargo space of the airplane. Instead of conveyor rollers, it is also possible to provide ball mats with rotatable balls near the loading opening, making it possible to convey in several directions. For this purpose, guiding elements are installed in addition.
For automatic or semi-automatic conveying, power drive units (PDUs) are also used. These drive units are normally located between the U-rails with the conveyor rollers or near the ball mats and consist essentially of a conveying wheel, a transmission, and a drive motor. The drive motor can be actuated manually by a loading foreman by a control stick or fully automatically by sensors.
In the loading or unloading process the containers or pallets are supported on the floor of the loading deck by the conveyor roller system. For continued controlled movement, a conveying wheel engages the container or pallet load from below with a friction coating against the bottom of the load. The conveying wheel and load are conveyed in the direction corresponding to the actuation and the direction of rotation of the drive motor. For continued movement in different directions, in particular for a deflection by 90.degree. in the area of the ball mats, drive units are known having housings which can be rotated by a controllable rotating mechanism in the floor of the loading deck. Furthermore, such a loading device comprises additional components, in particular, to secure the position of the loaded material.
The present invention relates to a drive unit which is suitable in particular for utilization in the above-described loading device. A known PDU drive unit of this type consists of a housing and a drive unit which is contained in the housing. The drive unit comprises a conveying wheel, a transmission and an electric, controllable drive motor. The conveying wheel protrudes from the housing with part of its circumferential surface. The housing must be installed on the floor of the loading deck in such manner that the upper part of the circumferential surface of the conveying wheel is located in the area of the loading plane defined by the conveyor rollers or ball mats.
The conveying wheel is constituted in this case by a rim with an inflatable rubber tire similar to a known vehicle wheel. The height of the rotational axis of the wheel is fixed. The tire pressure provides a contact pressure between the bottom of the load and the conveying wheel. When the load is very light, this obviously entails a risk that the load may be lifted by the tire pressure from the conveyor rollers or the ball mat, since the air tire is not sufficiently compressed. If this risk is to be excluded by reducing air pressure, there is the other danger, that the contact pressure supplied by the inflated tire no longer suffices. In the case of heavy loads, adhesive friction between the conveying wheel and the bottom load is not sufficient. The conveying wheel slips against the load without moving it forward. The adjustment of the air pressure requires extensive operator action. Since the friction between the conveying wheel and load must be adjusted generally without adjustment to the applicable weight conditions, the rubber surfaces of the conveyor wheel are subject to excess wear.
A drive unit of this type (German DE 39 11 214 C2), especially suitable for a loading device in an airplane, includes a conveying wheel, a transmission, and a drive motor, disposed within a housing. The transmission contains a planetary gear with a rotation-driven toothed sun gear, and at least one toothed wheel engaging the sun gear and serving as the planet gear. The planet gear is connected to the conveying wheel for a drive. It is also known to mount the planet gear on a swivelling planet gear carrier. Support elements are provided which interact with an associated stop in such manner that the conveying wheel is forced upward with a force that is proportional to the braking moment acting upon the conveying wheel.
Similar drive units operating on the same basic principle are furthermore known from German documents DE 39 42 381 C2, DE 42 24 818 A1 and DE 41 34 534 C1. In this case the planetary gear is designed so that the planet gear carrier is a swivelling holding device. Only one planet gear is rotatably mounted on the latter as an intermediate toothed wheel between the sun gear, which serves as the drive shaft and the inner toothing of the conveying wheel. The drive shaft as well as the drive motor is fixedly installed across the housing. The axial arrangement is eccentric. The axes of rotation of the drive shaft of the conveying wheel and of the intermediate toothed wheel, although parallel, are offset in relation to each other. The swivelling holding device is mounted around the fixed drive shaft. When the drive motor is switched on, the swivelling holding device is swivelled by a slip clutch serving as a braking arrangement. The swivelling holding device is moved upward by the eccentric arrangement of the intermediate toothed wheel and the conveying wheel. As the conveying wheel is applied against the lower surface of a load, the coupling slides through and the conveying wheel is rotated.
In this arrangement the force which brings the conveying wheel upward into contact with the conveyed load is relatively strong. The force depends in a non-linear manner on the existing swivelling angle of the swivelling holding device, since the angle from the vertical must be kept relatively small and since a kind of elbow lever arrangement is provided. With heavy load objects, the weight swivelling back causes this angle to be enlarged, causing the contact pressure to be reduced. This effect unfavorably counteracts the other effect, which causes the contact pressure to increase when the braking moment exerted on the conveying wheel increases. Furthermore, it is a considerable disadvantage that the load weight bears directly on the drive shaft by the conveying wheel and the intermediate wheel. This causes the latter to be subjected to rotational as well as bending loads with the danger of damage in case of heavy loads.
Accordingly, an object of the present invention is to provide an improved drive unit of the type described above having a more favorable application of forces.